Network Troubleshooting Lab for A+

Build a VirtualBox lab to practice CompTIA A+ networking commands.

Introduction

30 Second Summary

When your internet stops working, the fix usually comes down to a few specific commands and a systematic approach to finding the problem. These are the exact skills tested on the CompTIA A+ certification exams.

In this project, you will build a network troubleshooting lab using VirtualBox and Ubuntu Server on your Windows PC. You'll configure different network modes, practice the Linux and Windows networking commands that appear on the exam, and apply the CompTIA troubleshooting methodology to diagnose and fix real connectivity issues.

What You'll Build

A fully configured network troubleshooting lab with an Ubuntu Server virtual machine running multiple network configurations, giving you hands-on practice with the commands and concepts that make up the highest-weighted A+ exam domains.

By the end of this project, you'll have:

  • A working Ubuntu Server VM running inside VirtualBox with NAT and Internal Network configurations.
  • Practiced the exact Linux and Windows networking commands tested on CompTIA A+ Core 1 and Core 2.
  • A reusable troubleshooting lab environment you can return to for continued exam preparation.
  • Secret Mission: Set up a second VM and establish communication between two virtual machines on an Internal Network.

Are there any prerequisites?

No prior experience is required. This guide includes full setup instructions for all tools. You just need a Windows PC with at least 8 GB of RAM and 30 GB of free disk space.

Install VirtualBox and Download Ubuntu Server

To build your network troubleshooting lab, you need two things: software that can create virtual machines, and an operating system to install on them.

In this step, you will install VirtualBox (the virtualization software) and download Ubuntu Server (the Linux OS you will run inside your virtual machine). You will also confirm that your computer supports the hardware virtualization required to run VMs.

In this step, get ready to:
  • Download and install VirtualBox on your Windows PC.
  • Download the Ubuntu Server 24.04 LTS ISO file.
  • Verify your system supports hardware virtualization.
Download and install VirtualBox

VirtualBox is free, open-source software from Oracle that lets you run entire operating systems inside your current OS. This is called client-side virtualization, one of the key topics on the CompTIA A+ Core 1 exam.

First, check whether VirtualBox is already on your machine.

  • Press the Windows key to open your search bar.
  • Type VirtualBox and look for a result called Oracle VM VirtualBox.

✔️ I see VirtualBox 7.2 or higher

If VirtualBox Manager opens and you see version 7.2 or higher in the title bar or under Help > About VirtualBox, you are good to go. Skip ahead to the next substep (downloading Ubuntu Server).

ⓧ I see an older version of VirtualBox

If your installed version is below 7.2, you should upgrade to avoid compatibility issues with Ubuntu Server 24.04.

  • Open your browser and navigate to https://www.virtualbox.org/wiki/Downloads.
  • Click Windows hosts to download the installer file VirtualBox-7.2.8-173730-Win.exe (approximately 170 MB).
  • Run the downloaded installer.
  • Accept all default options by clicking Next through each screen.
  • When prompted about network interfaces, click Yes. Your internet will briefly disconnect while VirtualBox installs its network drivers.
  • Click Finish when the installation completes.
  • Open VirtualBox Manager to confirm it launches correctly.

What about my old VMs?

Upgrading VirtualBox preserves any existing virtual machines. Your VMs and settings carry over to the new version.

ⓧ VirtualBox is not installed

No worries. Follow the steps below to install VirtualBox from scratch.

  • Open your browser and navigate to https://www.virtualbox.org/wiki/Downloads.
  • Click Windows hosts to download the installer file VirtualBox-7.2.8-173730-Win.exe (approximately 170 MB).
  • Run the downloaded installer.
  • Accept all default options by clicking Next through each screen.
  • When prompted about network interfaces, click Yes. Your internet will briefly disconnect while VirtualBox installs its network drivers.
  • Click Finish when the installation completes.
  • Open VirtualBox Manager to confirm it launches correctly. You should see an empty list with a welcome message.

VirtualBox installer won't run?

Right-click the installer file and select Run as administrator. Some Windows security settings block installations unless you explicitly grant admin permissions.

help me troubleshoot VirtualBox installation issues on Windows

What is VirtualBox?

VirtualBox lets you create virtual machines (VMs) on your physical computer. Each VM behaves like a separate computer with its own operating system, network adapters, and storage.

This is the same technology tested in the Virtualization and Cloud Computing domain of the A+ Core 1 exam (11% of the exam weight).

Download Ubuntu Server 24.04 LTS ISO

Now you need the operating system to install inside your VM. Ubuntu Server is a Linux distribution that runs entirely from the command line, which is perfect for practicing the Linux CLI commands on the A+ Core 2 exam.

  • Open your browser and navigate to https://releases.ubuntu.com/noble/.
  • Find and click the link for ubuntu-24.04.4-live-server-amd64.iso to start the download (approximately 3.2 GB).
  • Save it to your Downloads folder. The download may take several minutes depending on your internet speed.

What is an ISO file?

An ISO file is a complete copy of a disc (like a DVD) stored as a single file. VirtualBox will mount this ISO as a virtual DVD drive so your VM can boot from it and install the operating system.

Why Ubuntu Server instead of Desktop?

Ubuntu Server does not include a graphical interface. This forces you to use the command line exclusively, which directly trains the Linux CLI skills tested on the CompTIA A+ Core 2 exam.

Verify your system supports virtualization

Virtual machines require a CPU feature called hardware virtualization (Intel VT-x or AMD-V). Most modern computers have this enabled, but you should confirm before proceeding.

  • Press the Windows key to open your search bar.
  • Type Command Prompt and press Enter to open it.
  • Check your system's virtualization support by running this command:
systeminfo
  • Scroll down to the bottom of the output and look for the section called Hyper-V Requirements.
  • Find the line that says Virtualization Enabled In Firmware and confirm it shows Yes.

✔️ It shows Yes

Your system is ready for virtualization. You can proceed to the next step.

ⓧ It shows No

You need to enable virtualization in your BIOS/UEFI settings. The exact steps vary by manufacturer, but the general process is:

  • Restart your computer.
  • Press the BIOS key during startup (commonly F2, F10, Del, or Esc). Your manufacturer's logo screen usually shows which key to press.
  • Look for a setting called Intel VT-x, AMD-V, Virtualization Technology, or SVM Mode. It is typically found under Advanced or Security settings.
  • Enable the setting and save your changes (usually F10 to save and exit).
  • After your computer restarts, open Command Prompt and run systeminfo again to confirm it now shows Yes.

Can't find the BIOS setting?

Search online for your computer's manufacturer name followed by "enable virtualization in BIOS" for model-specific instructions.

help me find and enable virtualization in my BIOS settings

ⓧ I don't see Hyper-V Requirements

If the Hyper-V Requirements section does not appear, your version of Windows may not display it. You can still proceed with the project. VirtualBox will show a clear error if virtualization is not available when you try to start your VM in the next step.

What does systeminfo tell you?

The systeminfo command displays detailed configuration information about your computer and its operating system. It covers hardware specs, network configuration, and virtualization capabilities. This is one of the Windows commands tested on the A+ exam.

Your tools are downloaded and ready. Next up, you will create your first virtual machine and install Ubuntu Server on it.

Create and Configure Your Virtual Machine

You have VirtualBox installed and your Ubuntu Server ISO ready to go.

Now it is time to create your first virtual machine and install an operating system on it. A virtual machine is a simulated computer running inside your real computer. You will configure it with specific hardware settings (RAM, CPU, disk space) and then install Ubuntu Server from the ISO file you downloaded.

In this step, get ready to:
  • Create a new VM in VirtualBox with the correct settings.
  • Install Ubuntu Server 24.04 LTS on the VM.
  • Log in and verify your VM works with Linux commands.
Create a new VM in VirtualBox

VirtualBox uses a wizard to walk you through creating a new virtual machine. You will set the name, operating system type, memory, and disk size.

  • In VirtualBox Manager, click New in the toolbar at the top.
  • The Create Virtual Machine wizard opens. In the Name field, type A-Plus-Lab.
  • Set Type to Linux.
  • Set Subtype to Ubuntu.
  • Set Version to Ubuntu (64-bit).
  • Check the Skip Unattended Installation checkbox. You will install the OS manually so you can learn the process.
  • Click Next to continue to the Hardware page.
  • Set Base Memory to 2048 MB (2 GB).
  • Set Processors to 1 CPU.
  • Click Next to continue to the Virtual Hard Disk page.
  • Select Create a Virtual Hard Disk Now.
  • Set the disk size to 25 GB.
  • Click Next to see the Summary page.
  • Review your settings and click Finish.

Your new A-Plus-Lab VM now appears in the Machines list on the left side of VirtualBox Manager.

What is a VDI disk?

VirtualBox creates a VDI (Virtual Disk Image) file that acts as the VM's hard drive. The "dynamically allocated" setting means the file starts small and only grows as you add data, saving space on your real hard drive.

Don't see Ubuntu (64-bit) in the Version dropdown?

If only 32-bit options appear, virtualization may not be fully enabled. Return to Step 1 and confirm that "Virtualization Enabled In Firmware" shows Yes in the systeminfo output. You may need to enable Intel VT-x or AMD-V in your BIOS/UEFI settings.

help me troubleshoot why VirtualBox only shows 32-bit options

Attach the Ubuntu ISO and install the OS

With the VM created, you need to load the Ubuntu Server ISO into the VM's virtual DVD drive and boot from it. The Ubuntu Server installer is entirely text-based. You navigate it with the arrow keys and Enter.

  • Select your A-Plus-Lab VM in the Machines list and click Settings.
  • Click Storage in the left panel.
  • Under Controller: IDE, click the Empty disk icon.
  • On the right side, click the small disk icon next to Optical Drive and select Choose a disk file.
  • Navigate to your Downloads folder and select ubuntu-24.04.4-live-server-amd64.iso.
  • Click OK to close the Settings window.
  • Click Start in the toolbar to boot the VM.

What happens when you click Start?

The VM boots from the ISO file just like a physical computer boots from a USB drive. A new window opens showing the VM's display. You will see the Ubuntu Server installer start up after a few moments.

The installer will walk you through several screens. Follow these steps in the VM window:

  • Select your language (English) and press Enter.
  • If the installer offers to update itself, select Continue without updating.
  • Select your keyboard layout and press Done.
  • On the network configuration screen, accept the defaults and select Done.
  • On the proxy screen, leave it blank and select Done.
  • On the mirror screen, accept the default and select Done.
  • On the storage screen, leave "Use an entire disk" checked and select Done.
  • Review the storage layout summary and select Done.
  • When asked to confirm destructive action, select Continue.
  • On the Profile screen, enter your name, server name, username, and password. Remember these credentials.
  • On the Ubuntu Pro screen, select Skip for now and then Done.
  • On the SSH Setup screen, check Install OpenSSH server and select Done.
  • On the Featured Server Snaps screen, do not select anything. Select Done.
  • Wait for the installation to complete. You will see log messages scrolling by. This may take several minutes.
  • When you see Reboot Now at the bottom, press Enter.

Why install OpenSSH server?

OpenSSH lets you connect to the server remotely over the network. It is explicitly listed in the CompTIA A+ exam objectives under remote access technologies. Installing it now means your lab is ready for SSH practice later.

VM window captured your mouse?

VirtualBox captures your keyboard and mouse when you click inside the VM window. Press the Right Ctrl key (the Host Key shown at the bottom-right of the VM window) to release your mouse back to Windows.

help me understand VirtualBox Host Key and mouse capture

Log in and verify your VM works

After the reboot, VirtualBox automatically ejects the ISO so your VM boots from its virtual hard disk. You should see a login prompt on a black screen.

  • Enter the username you created during installation and press Enter.
  • Enter your password and press Enter. The characters will not appear on screen as you type. This is normal Linux security behaviour.

You should see a command prompt showing your username and server name. Now verify everything works by running two commands.

  • Confirm your machine name by running this command:
hostname
  • You should see the server name you entered during installation.
  • View your Linux kernel information by running:
uname -a

You should see output containing "Ubuntu" and the kernel version number. This confirms your VM is running Ubuntu Server successfully.

What do these commands do?

`hostname` displays the name of your machine. This is the same name you entered during installation.

`uname -a` displays detailed system information including the operating system, kernel version, and architecture. Both commands appear on the CompTIA A+ Core 2 exam.

Seeing a blank screen after reboot?

Sometimes the VM display goes to sleep. Press Enter once or twice inside the VM window. The login prompt should appear.

If you see a message about removing installation media, just press Enter. VirtualBox handles the ISO ejection automatically.

help me troubleshoot my Ubuntu Server VM not showing a login prompt after reboot

Your virtual machine is up and running with Ubuntu Server installed. Next up, you will explore the network configuration and practice the exact networking commands that appear on the CompTIA A+ exam.

Explore Network Configuration with NAT Mode

Your VM is running and you're logged in. Now it's time to explore how your virtual machine connects to the internet.

VirtualBox's default NAT mode gives your VM internet access through your host machine. You'll use the Linux and Windows networking commands tested on the CompTIA A+ exam to examine this setup.

In this step, get ready to:
  • Examine your VM's IP address, gateway, and DNS configuration.
  • Test internet connectivity and DNS resolution using exam commands.
  • Compare your VM's network with your Windows host's network.
Check your current network configuration

Your VM is currently using NAT mode, which is VirtualBox's default. This means VirtualBox acts as a router between your VM and the internet, assigning your VM its own private IP address via DHCP.

  • In your VM terminal, display all network interfaces and their IP addresses by running:
ip addr show
  • Display the routing table including the default gateway by running:
ip route show
  • Check your DNS server configuration by running:
cat /etc/resolv.conf

What should I see?

Your ip addr show output shows the interface enp0s3 with an IP address of 10.0.2.15/24. This is the default IP that VirtualBox assigns to the first guest in NAT mode.

Your ip route show output shows the default gateway as 10.0.2.2. This is VirtualBox's virtual router that forwards traffic to your host's real network.

Your /etc/resolv.conf file shows a nameserver entry (typically 10.0.2.3). This is VirtualBox's DNS proxy that forwards lookups to your host's DNS servers.

Not seeing an IP address on enp0s3?

If enp0s3 shows no IP address or only a link-local address starting with 169.254, VirtualBox's DHCP may not have assigned one. Try running sudo dhclient enp0s3 to request a new address, then check again with ip addr show.

Also confirm your VM's network adapter is set to NAT in VirtualBox. Shut down the VM, open Settings > Network, and verify Adapter 1 says NAT.

Still stuck? Help me troubleshoot why my VM has no IP address in NAT mode.

Test connectivity using exam-relevant commands

Now that you know your IP configuration, it's time to test whether your VM can actually reach the internet. The commands below are all tested on the CompTIA A+ exam.

  • Test basic connectivity to Google's DNS server by running:
ping -c 4 8.8.8.8
  • Test DNS resolution and connectivity together by running:
ping -c 4 google.com

Why test with an IP address AND a hostname?

Pinging 8.8.8.8 tests raw network connectivity only. If this succeeds but pinging google.com fails, you know the network works but DNS resolution is broken.

This is a key troubleshooting technique on the A+ exam. It lets you isolate whether a connectivity problem is a network issue or a DNS issue.

  • Perform a detailed DNS lookup by running:
dig google.com
  • Confirm full internet access by updating your package lists:
sudo apt-get update

You should see both ping commands return replies with round-trip times. The dig command returns DNS records including the resolved IP address. The apt-get update command downloads package list updates from the internet, confirming your VM has full connectivity.

What does dig show?

The dig command shows the full DNS lookup process. Look for the ANSWER SECTION which contains the IP address that google.com resolves to. You will also see which DNS server answered the query and how long it took.

Ping commands failing?

If ping 8.8.8.8 fails, your VM cannot reach the internet at all. Verify that your VM's adapter is set to NAT in VirtualBox settings, and confirm your host machine has a working internet connection.

If ping 8.8.8.8 succeeds but ping google.com fails, DNS resolution is the problem. Check /etc/resolv.conf has a valid nameserver entry.

Need more help? Help me figure out why my VM can't reach the internet in NAT mode.

Compare with Windows host networking

Now you'll run similar commands on your Windows host to see how the two networks differ. This comparison helps you understand what NAT mode actually does.

  • Switch to your Windows desktop (click outside the VM window or press the Host Key shown in the bottom-right corner of the VirtualBox window).
  • Press the Windows key, type cmd, and press Enter to open Command Prompt.
  • Display your host's full network configuration by running:
ipconfig /all
  • Test connectivity from your host by running:
ping 8.8.8.8
  • Perform a DNS lookup from Windows by running:
nslookup google.com

What is the difference between these two networks?

Your Windows host has an IP address in the 192.168.x.x range (assigned by your home router). Your VM has an IP in the 10.0.2.x range (assigned by VirtualBox's virtual DHCP server).

NAT mode creates a completely separate private network for your VM. VirtualBox translates traffic between the two networks, which is exactly what a real NAT router does. Your VM shares your host's internet connection without being directly visible on your home network.

Note the key equivalents between platforms. Linux uses ip addr show where Windows uses ipconfig /all. Linux uses dig where Windows uses nslookup. Both platforms use ping. The A+ exam tests you on all of these.

You've now used the core networking commands from both the Linux and Windows sides of the A+ exam. Next up, you'll switch your VM to Internal Network mode and practice troubleshooting when things break.

Configure Internal Network Mode and Troubleshoot

In the previous step, you confirmed your VM can reach the internet through NAT mode. Every command worked because VirtualBox was handling IP assignment, DNS, and routing for you automatically.

Now you are going to break that connectivity on purpose. By switching to Internal Network mode, your VM will be completely isolated from both the host and the internet. This creates the perfect troubleshooting scenario because there is no DHCP server to assign an IP address, so you must diagnose the problem and fix it manually.

In this step, get ready to:
  • Switch your VM from NAT to Internal Network mode and observe the broken network.
  • Apply the CompTIA troubleshooting methodology to diagnose and fix the connectivity issue.
  • Practice additional A+ exam commands on the configured interface.
Change the network adapter to Internal Network

To create an isolated network with no automatic configuration, you need to change the adapter mode in VirtualBox settings. The VM must be shut down first.

  • Shut down your VM by running this command:
sudo shutdown now

Your VM window will close and you will return to the VirtualBox Manager.

  • In VirtualBox Manager, select your A-Plus-Lab VM and click Settings.
  • Go to Network.
  • On Adapter 1, change the Attached to dropdown from NAT to Internal Network.
  • In the Name field below, type LabNet.
  • Click OK.
  • Click Start to boot the VM again.
  • Log back in with the username and password you created during installation.

What is Internal Network mode?

Internal Network mode creates a software-based network that is only visible to VMs on the same host that share the same network name. It is completely isolated from both your host machine and the internet.

Unlike NAT mode, there is no built-in DHCP server on an Internal Network by default. This means VMs will not automatically receive an IP address unless you configure one manually.

VM taking a long time to boot?

Ubuntu may wait up to 2 minutes trying to get an IP address from a DHCP server that does not exist. This is normal on an Internal Network with no DHCP. Wait for the login prompt to appear.

help me troubleshoot my VM boot delay after switching to Internal Network

Observe the broken network and diagnose

Your VM now has no connectivity. This is intentional. You are going to practice the CompTIA troubleshooting methodology to identify and fix the problem step by step.

Step 1 of the methodology is to identify the problem. Check your network configuration by running:

  • Check your network interfaces by running this command:
ip addr show

Notice that the enp0s3 interface has no IP address assigned, or only shows a link-local address. This confirms the problem.

What does this tell you?

In NAT mode, VirtualBox's built-in DHCP server automatically assigned 10.0.2.15 to your VM. On the Internal Network, there is no DHCP server at all. The interface is up but has no way to get an address.

This is Step 2 of the methodology: establish a theory. Your theory is that there is no DHCP server on this Internal Network to assign an IP.

Step 3 is to test the theory by manually assigning an IP address. This proves the interface itself is working.

  • Assign a static IP address to your interface by running this command:
sudo ip addr add 192.168.10.1/24 dev enp0s3

What does this command do?

This assigns the IP address 192.168.10.1 with a /24 subnet mask (255.255.255.0) to the enp0s3 interface. The sudo prefix runs the command with administrator privileges, which is required for network configuration changes.

  • Confirm the IP is now assigned by running:
ip addr show enp0s3

You should see 192.168.10.1/24 listed under the enp0s3 interface. This confirms your static IP assignment worked.

Step 4 is to verify. Ping yourself to confirm the interface is fully functional.

  • Test that the interface is up by pinging yourself:
ping -c 2 192.168.10.1

You should see 2 packets transmitted and 2 received with 0% packet loss. This confirms the interface is up and the IP stack is working.

What about Step 5?

Step 5 of the CompTIA methodology is to document findings. Here is what you would document: Internal Network mode isolates VMs from the host and internet. Without a DHCP server or manual configuration, there is no connectivity. Assigning a static IP resolves the issue for local communication.

Ping shows 'Network is unreachable'?

Make sure you typed the command exactly as shown, including the /24 subnet mask. Run ip addr show enp0s3 to check that the IP was assigned correctly.

help me fix the Network is unreachable error when pinging my static IP on Internal Network

Practice additional exam commands on the configured interface

Now that your interface is configured, you can practice more Linux commands that appear on the CompTIA A+ Core 2 exam. Each of these commands is explicitly listed in the exam objectives.

  • Search for DNS configuration using grep by running:
grep -i "nameserver" /etc/resolv.conf

What does this command do?

The grep command searches text for matching patterns. The -i flag makes the search case-insensitive. This searches /etc/resolv.conf for any line containing the word "nameserver". You will see the DNS server entry from installation, but since you are on an Internal Network now, that DNS server is not reachable.

  • Check if a DHCP client process is running by combining ps and grep:
ps aux | grep dhcp

What does this command do?

The ps aux command lists all running processes. The pipe (|) sends that output to grep, which filters for lines containing "dhcp". This tells you whether the system is still trying to get an IP from a DHCP server that does not exist.

  • Practice disabling and re-enabling the network interface:
sudo ip link set enp0s3 down
  • Re-enable the interface by running:
sudo ip link set enp0s3 up
  • Verify the interface state changed by running:
ip addr show enp0s3

Why disable and re-enable?

Toggling an interface down and up is a common troubleshooting step. It resets the interface state without rebooting the entire machine. On the exam, you may need to identify the correct command to bring an interface back online.

Note that bringing the interface down may remove your manually assigned IP. If ip addr show enp0s3 no longer shows 192.168.10.1/24, re-assign it with the sudo ip addr add command from earlier.

  • Access the manual page for the ip command by running:
man ip

Press q to exit the manual page when you are finished reading.

What is man?

The man command opens the manual page for any Linux command. It is the built-in documentation system. On the A+ exam, you need to know that man is how you look up command syntax and options directly from the terminal.

You just experienced what happens when a network has no DHCP server and fixed it using manual configuration. Next up, you will apply the full CompTIA troubleshooting methodology to a simulated DNS failure.

Apply the Troubleshooting Methodology

In the last step, you switched to Internal Network mode and experienced a real connectivity failure. You diagnosed the missing DHCP server, manually assigned an IP, and practiced key exam commands.

Now you will apply the full CompTIA 7-step troubleshooting methodology to a simulated DNS failure. You will intentionally break your network configuration, then systematically diagnose and fix it.

In this step, get ready to:
  • Switch back to NAT mode and simulate a DNS failure.
  • Diagnose and fix the simulated issue using the CompTIA troubleshooting methodology.
  • Review all networking commands practiced across this project.
Switch back to NAT and create a documented troubleshooting scenario

To simulate a real-world troubleshooting scenario, you need internet connectivity first. You will switch the adapter back to NAT mode, confirm the network works, then intentionally break DNS to practice diagnosing issues.

  • Shut down your VM by running this command:
sudo shutdown now
  • In VirtualBox Manager, select your A-Plus-Lab VM and click Settings.
  • Go to Network.
  • On Adapter 1, change Attached to from Internal Network back to NAT.
  • Click OK, then click Start to boot the VM.
  • Log in with your username and password.
  • Confirm connectivity is restored by running:
ping -c 2 google.com

You should see successful replies, confirming your VM can reach the internet again through NAT mode.

Ping still failing after switching to NAT?

Your interface may still have the static IP from the Internal Network configuration. Run ip addr show enp0s3 to check. If you see 192.168.10.1 instead of 10.0.2.15, reboot the VM with sudo reboot to let DHCP assign a fresh IP.

Help me troubleshoot why my VM can't ping after switching back to NAT mode.

Now you will intentionally break the DNS configuration to create a troubleshooting scenario.

  • Open the DNS configuration file by running:
sudo nano /etc/resolv.conf
  • Find the line that starts with nameserver.
  • Change the IP address after nameserver to 192.168.99.99 (a non-existent DNS server).
  • The line should now read:
nameserver 192.168.99.99
  • Press Ctrl+O to save the file.
  • Press Enter to confirm the filename.
  • Press Ctrl+X to exit nano.

What is nano?

Nano is a command-line text editor that is explicitly listed on the CompTIA A+ Core 2 exam objectives. It is simpler than other Linux editors like vim, making it ideal for quick configuration changes.

Your DNS is now pointed at a server that does not exist. The network connection itself is fine, but your VM cannot translate domain names into IP addresses.

Diagnose and fix the simulated issue

The CompTIA A+ exam tests a specific 7-step troubleshooting methodology. You will now walk through every step of this methodology on a real (simulated) network problem.

Step 1: Identify the problem.

  • Attempt to reach a website by running:
ping google.com

You should see the error message: ping: google.com: Temporary failure in name resolution. This tells you the system cannot resolve the hostname to an IP address.

Step 2: Establish a theory of probable cause.

The error mentions name resolution, which points to DNS. Your theory is that DNS resolution is broken, but the underlying network connection might still work.

Step 3: Test the theory to determine the cause.

  • Test whether the network itself works by pinging an IP address directly (bypassing DNS):
ping -c 2 8.8.8.8

This command succeeds. It confirms the network connection works. Only DNS is broken. Your theory is correct.

Why does pinging an IP address prove the network works?

When you ping a hostname like google.com, the system must first resolve the name to an IP (using DNS), then send the ping. When you ping an IP address directly, you skip the DNS step entirely.

If a hostname ping fails but an IP ping succeeds, the problem is isolated to DNS. This is a key troubleshooting technique tested on the A+ exam.

Step 4: Establish a plan of action to resolve the problem.

The plan is straightforward. Fix the DNS configuration in /etc/resolv.conf by replacing the non-existent nameserver with a valid one.

Step 5: Implement the solution.

  • Open the DNS configuration file by running:
sudo nano /etc/resolv.conf
  • Change nameserver 192.168.99.99 to:
nameserver 8.8.8.8
  • Press Ctrl+O to save, then Ctrl+X to exit.

Why 8.8.8.8?

8.8.8.8 is Google's public DNS server. It is widely used as a quick replacement when troubleshooting DNS issues because it is reliable, fast, and easy to remember.

Step 6: Verify full system functionality.

  • Test that DNS resolution works again by running:
ping -c 2 google.com

You should see successful replies. Both name resolution and network connectivity are working.

Step 7: Document findings, actions, and outcomes.

In a real-world scenario, you would record your findings. Here is your documentation for this troubleshooting exercise:

  • Problem: DNS resolution failed. Pinging hostnames returned "Temporary failure in name resolution."
  • Root cause: The /etc/resolv.conf file contained an invalid nameserver address (192.168.99.99).
  • Fix: Replaced the invalid nameserver with 8.8.8.8 (Google Public DNS).
  • Verification: Successfully pinged google.com after applying the fix.

Ping still failing after fixing resolv.conf?

Make sure you saved the file correctly in nano. Open it again with sudo nano /etc/resolv.conf and verify the line reads exactly nameserver 8.8.8.8 with no extra spaces or typos.

Help me fix DNS resolution on my Ubuntu Server VM after editing resolv.conf.

Review all commands practiced (exam reference)

Throughout this project, you have practiced commands from both the CompTIA A+ Core 1 and Core 2 exam objectives. Here is your complete reference.

Linux commands covered (Core 2 - Operating Systems domain):

  • ip - network interface configuration.
  • ping - test connectivity.
  • dig - DNS lookup.
  • grep - search text.
  • ps - list processes.
  • sudo - run as administrator.
  • apt-get - package management.
  • cat - display file contents.
  • nano - text editor.
  • man - manual pages.
  • hostname - display system hostname.
  • uname - display system information.
  • shutdown - power off the system.

Windows commands covered (Core 1 and Core 2):

  • ipconfig /all - display network configuration.
  • ping - test connectivity.
  • nslookup - DNS lookup.
  • systeminfo - display system information.

Keep this as a study reference

These 13 Linux commands and 4 Windows commands map directly to the A+ exam objectives. You can return to this lab anytime to practice them again before your exam.

What are the 7 steps of CompTIA's troubleshooting methodology?

  • Identify the problem.
  • Establish a theory of probable cause.
  • Test the theory to determine the cause.
  • Establish a plan of action to resolve the problem.
  • Implement the solution.
  • Verify full system functionality.
  • Document findings, actions, and outcomes.

This methodology appears in both Core 1 (Hardware and Network Troubleshooting) and Core 2 (Software Troubleshooting). Memorize the order and be ready to identify which step applies in scenario-based questions.

Secret mission

Build a Two-VM Network

Ready to level up? Clone your VM, configure static IPs on both machines, and establish communication between them on the Internal Network. This proves you understand IP addressing, subnet configuration, and can troubleshoot host-to-host connectivity.

Clean Up Your Resources

Clean Up Your Resources

This project runs entirely on your local machine, so there are no ongoing cloud costs. Decide whether to keep your resources running, pause them to come back later, or delete them entirely.

Resources you used:

  • A-Plus-Lab VM (VirtualBox virtual machine).
  • A-Plus-Lab-2 VM (VirtualBox virtual machine, Secret Mission).
  • Ubuntu Server 24.04 LTS ISO file (downloaded to your computer).
  • Oracle VirtualBox application (installed on your Windows PC).

Keep everything running

No action needed. Choose this if you plan to keep practising networking commands or want to return to your lab environment for exam prep.

  • Your VMs and VirtualBox stay installed and ready to use anytime.
  • The VMs only use disk space when powered off. They do not consume CPU or memory until you start them.
  • The Ubuntu Server ISO can stay in your Downloads folder for future use.

Pause - I'll come back to this later

Shut down your VMs to free up memory and CPU. Your VM files stay on disk so you can start them again later.

  • If either VM is still running, log in and run sudo shutdown now on each one.
  • Close the VirtualBox Manager window.
  • Your VMs are saved on disk and will appear in VirtualBox Manager next time you open it.

Delete - I don't want to use this again

Remove all project resources and reclaim disk space. Follow these steps in order.

  • Make sure both VMs are powered off. If either is running, log in and run sudo shutdown now.
  • In VirtualBox Manager, right-click A-Plus-Lab-2 and select Remove.
  • In the confirmation dialog, choose Delete all files to permanently delete the VM and its virtual hard disk.
  • Right-click A-Plus-Lab and select Remove. Choose Delete all files again.
  • Delete the Ubuntu Server ISO file (ubuntu-24.04.4-live-server-amd64.iso) from your Downloads folder.
  • To uninstall VirtualBox, open Settings on your Windows PC, select Apps, then Installed apps.
  • Search for Oracle VirtualBox, click the three-dot menu, and select Uninstall.
  • After uninstalling, delete the leftover VirtualBox data folder at C:\Users\<your-username>\VirtualBox VMs if it still exists.

Nice Work!

Nice Work!

You just built a fully functional network troubleshooting lab using VirtualBox and Ubuntu Server. You now have a reusable environment for practicing the hands-on networking and Linux skills that appear on the CompTIA A+ exam.

You've learned how to:

  • Create and configure a virtual machine in VirtualBox, switching between NAT and Internal Network modes to simulate different network environments.
  • Practice 13+ Linux commands and 4 Windows networking commands explicitly listed in the CompTIA A+ Core 1 and Core 2 exam objectives.
  • Apply the CompTIA 7-step troubleshooting methodology to diagnose and fix a real DNS resolution failure.
  • Secret Mission: Establish host-to-host communication between two VMs on an Internal Network, demonstrating IP addressing and subnet configuration skills.

Ready to quiz yourself?